Chlorine / Cl₂

There are two chlorine atoms in a dichlorine molecule. Discovered by Carl Wilhelm Scheele in 1774, it is a yellow-green gas under normal temperature and pressure conditions (khlôros means "green" in Greek).
It is 2.5 times denser than air. Its odour is suffocating and very unpleasant and it is extremely toxic because it recombines with the moisture in the mucous membranes and in the lungs to create acids which attack the tissues. During the First World War it was used as a warfare gas, known as bertholite.
Dichlorine is soluble in water creating chlorine water, but it is not soluble in salt water. We often mistakenly refer to aqueous sodium hypochlorite solutions in highly alkaline form as "liquid chlorine", which are in fact either 6% bleach or 12% bleach. We also often hear the expression "solid chlorine" to refer to calcium hypochlorite crystals or to trichloroisocyanuric acid (chlorine tablets for swimming pools).

Flammability: Chlorine itself is not flammable but it will support combustion, as it is a very reactive oxidising agent. Chlorine reacts violently (it can even catch fire or explode) with several organic compounds and combustibles, as well as with phosphorus, arsenic, antimony and metals in powder form. There is also a risk of sudden fire when it is in contact with grease, oil and silicone.
Explosiveness: Mixtures of chlorine with hydrogen, acetylene, ethane, ethylene and ammonia can explode if exposed to a spark, light or certain catalysts.